I think a bit about this, and I invent a method of using bitcoin notes in close to usual banknotes way.
You need 3-th side in this, say paperbitcoins.com
You need to create an account on paperbitcoins.com and preload it with some bitcoins on you'r main wallet
Next you need to create a bills what you need. Say 500 mBTC, 1 BTC, 1 BTC, 2 BTC, 2 BTC, 5 BTC, 10 BTC.
Then you got the barcodes, related to this bills. It's not bitcoin keys, it's paperbitcoins.com-inside codes.
Then you need to print the notes like this
in my example it will be 7 notes with nominals 0.5, 1, 1, 2, 2, 5 and 10 BTC. Probably you need to use a different colors for different face value for usability reasons, but it's not matter. You need you'r barcodes on that notes. Actually paperbitcoins.com can automatically generate for you 7 svg files with you'r notes.
Then you need a RSA SecurID token, connected to you account on paperbitcoins.com.
You also need the public key from your main wallet
for your change. You can paint it on your RSA token for example.
Say, you need to pay 2,9 BTC.
You find 1 BTC and 2 BTC notes in your wallet and give it, with your RSA token, to the seller.
Then the seller scan codes from your notes, enter the authentication code from your RSA token (in software from paperbitcoins.com), and got 3 BTC from you. Then the seller scan public key from the main wallet
and charge you 0.1 BTC back. You can't control you charge back, but if you care, you can print a many small notes, like 50 mBTC and pay without a charge.
Then the seller gives you back your notes and RSA token. This is a reusable notes.
When the authentication code on the RSA token will change, or after any amount of time, you choose, more than this, paperbitcoins.com automatically will reload your notes from the main wallet
. And now you can use it again.
This is a way more secure than a notes with bitcoin public keys on it. You do not need to hide and bother about your notes.
And my grandma still can use it. I can print a notes for her, give her a token, and tell how to use it.
This is similar to common money, but you need to take it back after you use it.
If you loose it, no one can stole your money, without your token. If you forgot to take you notes back, don't worry, your money is not be lost, you can go to paperbitcoins.com, destroy that notes, take money from them on you main wallet
and create new notes.
BTW, I don't know if this is usable, but what do you think?